Mass Extinction

Mass Extinction Events Can Turn Freshwater Into Toxic Soup, And It's Already Happening

Apart from the global catastrophe that killed off most of the dinosaurs, some experts think almost all the mass extinctions in Earth's history were followed by a proliferation of microbes in rivers and lakes.
Human activity is leading to a similar mass extinction event today.
We're seeing more and more toxic algae blooms in lakes and in shallow marine environments that's related to increases in temperature and changes in plant communities which are leading to increases in nutrient contributions to freshwater environments.
The volcanism was a source of CO2 in the past, but we know that the rate of CO2 input that was seen back then was similar to the rate of CO2 increases we're seeing today because of anthropogenic effects.
Algae and bacteria are normal parts of a healthy freshwater environment, but sometimes they can grow out of control and deplete the water of oxygen, creating 'dead zones'.
The persistent dead zones prevented the reestablishment of important carbon sinks, like peatlands, and slowed down climate and ecosystem recovery.
This tends to happen with global warming, deforestation,wildfire, and the rush of soil nutrients into waterways, which can feed microbes. All three of these factors are in play today, which is why we are probably seeing increases in toxic blooms already.

The good news is that this time many of the changes are in our control. The bad news is that whatever happens next is our own fault.

Reference: sciencealert.com